1. Field of the Invention
Apparatuses and methods consistent with the present invention relate to a radio frequency identification (RFID) reader and a data recovery method which selectively selects only an integrity-checked phase signal, when receiving a backscattered signal of an RFID tag via an in-phase (I) signal path or a quadrature-phase (Q) signal path, and may reduce a shadow area of a read range.
2. Description of the Related Art
When a radio frequency identification (RFID) reader receives a backscattered signal of an RFID tag in a general RFID system, a phase delay occurs according to a distance between the RFID reader and the RFID tag. When a receiver of the RFID reader is demodulated by using a quadrate mixer, the backscattered signal of the RFID tag is received via an I signal path or a Q signal path due to the phase delay.
The received signal is recovered as original data in a predetermined recovery device. Also, through the recovery operations described above, information included in the RFID tag may be read by the RFID reader.
As an example of a conventional data recovery method, a system of recovering a radio frequency (RF) signal which is received via an I channel and a Q channel is disclosed in U.S. Pat. No. 6,122,329 (hereinafter, patent '329) entitled ‘Radio frequency identification interrogator signal processing system for reading moving transponders’.
The patent '329 discloses a data recovery method in which an I signal and a Q signal are received, a phase angle between the I signal and the Q signal is estimated, and data is recovered by combining the I signal and the Q signal.
However, in this data recovery method, an RFID reader may not recover a phase of the I signal or the Q signal, when performing a direct current (DC)-cancellation by using a high-pass filter (HPF) in a receiver of an RFID interrogator, i.e. an RFID reader.
Specifically, the RFID reader which is embodied in the patent '329 may not distinguish an X radian and an X+π radian. In the following equations, a difference between a recovered signal and a signal prior to being transmitted is shown.
            I      ⁡              (        t        )              ≅          A      ·              (                                                                        m                ⁡                                  (                  t                  )                                            ⁢              cos              ⁢                                                          ⁢              φ                                            -                                                                cos                ⁢                                                                  ⁢                φ                            2                                                  )                                Q        ⁡                  (          t          )                    ≅                        A          ·                                    (                                                                                                            m                      ⁡                                              (                        t                        )                                                              ⁢                    sin                    ⁢                                                                                  ⁢                    φ                                                                    -                                                                                              sin                      ⁢                                                                                          ⁢                      φ                                        2                                                                                )                        .                                                  ⁢                                                                                                      I                      2                                        ⁡                                          (                      t                      )                                                        +                                                            Q                      2                                        ⁡                                          (                      t                      )                                                                      ≠                                                    ⁢                  m          ⁡                      (            t            )                                ,  
where m(t) designates a normalized ideal Rx signal, and the  designates a phase of m(t).
In this instance, as shown above, the recovered signal and the signal prior to be transmitted may be different.
As another example of a conventional data recovery method, a method of deriving data bits in order to recover a tag signal is disclosed in U.S. Pat. No. 6,501,807 (hereinafter, patent '807) entitled ‘Data recovery system for radio frequency identification interrogator’.
In the patent '807, a tag signal is received, an over-sampling is performed, relative polarities are compared, and data bits are derived.
However, this data recovery method may cause a partial shadow area of a read range, when selecting any one of the I signal and the Q signal based on a relative size of the I signal and the Q signal.
Specifically, in the conventional method disclosed in the patent '807, when magnitudes of the I signal and the Q signal are similar, and signs of cos  and sin  are opposite, there appears a shadow area in which a tag signal may not be read at a distance of 3π/4 and a distance of 7π/4.
Accordingly, a new data recovery model which enables a reliable recovery of tag information which is transmitted from an RFID tag and significant reduction of a shadow area in a read range is highly required.